A vehicle has parking sensors to detect an obstacle behind the vehicle. The parking sensors determine a distance of the vehicle from the obstacle using ultrasonic signals when backing a vehicle. The parking sensor operates at ultrasonic frequencies. The parking sensor outputs an ultrasonic detecting signal to detect whether any obstacle is behind the rear of the vehicle and receives an ultrasonic signal as reply from the obstacle.
A vehicle generally requires multiple parking sensors to cover the entire rear of the vehicle which makes it a cost intensive solution. Also, the ultrasonic parking sensors use a time division obstacle detecting method in which each sensor sends and receive ultrasonic detect signal in a defined time slot. Thus, the process of detecting obstacles using ultrasonic sensors is time consuming which is unsafe in vehicles moving with high velocity.
Ultrasonic parking sensors require the measurement and drilling of boles in the vehicle's bumper to install transducers. There are risks associated with drilling and mounting the transducers into the bumper. The performance of the ultrasonic sensors is sensitive to temperature and atmospheric conditions such as snow and rain. The performance of ultrasonic sensors is severely degraded when the sensors are covered with snow. In addition, the distance over which the ultrasonic sensors operate is limited.
The use of radars in automotive applications is evolving rapidly. Radars do not have the drawbacks discussed above in the context of ultrasonic sensors. Radar finds use in number of applications associated with a vehicle such as collision warning, blind spot warning, lane change assist, parking assist and rear collision warning. Pulse radar and FMCW (Frequency Modulation Continuous Wave) radar are predominately used in such applications.
In the pulse radar, a signal in the shape of a pulse is transmitted from the radar at fixed intervals. The transmitted pulse is scattered by the obstacle. The scattered pulse is received by the radar and the time difference between the start of transmission of the pulse and the start of reception of the scattered pulse is proportional to a distance of the radar from the target. For better distance resolution, a narrower pulse is used which requires a high sampling rate in an ADC (analog to digital converter) used in the pulse radar. In addition, sensitivity of a pulse radar is directly proportional to the power which complicates the design of the pulse radar.
In an FMCW radar, a transmit signal is frequency modulated to generate a ramp segment. A plurality of obstacles scatters the ramp segment to generate a scattered signal. The scattered signal is received by the FMCW radar. A signal obtained by mixing the ramp segment and the scattered signal is termed as an IF (intermediate frequency) signal. The frequency (f) of the IF signal is proportional to the distance (d) of the obstacle from the FMCW radar and also to the slope (S) of the ramp segment.
Distance resolution defines the capability of the FMCW radar to resolve closely spaced objects or obstacles. For a given duration of the ramp segment, the distance resolution is directly proportional to the slope of the ramp segment. A maximum frequency of the IF signal is proportional to the product of the distance of a farthest obstacle which can be detected by the FMCW radar and the slope of the ramp segment. Further, a maximum measurable velocity of an obstacle by the FMCW radar is proportional to a repetition rate of the ramp segments in a frame which is dependent on the duration of each ramp segment.
The IF signal is sampled by an analog to digital converter (ADC). A sampling rate of the ADC is proportional to the maximum frequency of the filtered IF signal. Thus, if a large distance and a high resolution are required to be supported by the FMCW radar, it will require the maximum frequency of the IF signal to be higher and subsequently a higher sampling rate of the ADC. The high sampling rate of the ADC also increases the processing requirement of a DSP (digital signal processor) in the FMCW radar. Thus it would be useful to have techniques which can work with a limited IF bandwidth but yet allow the FMCW radar to detect one or more obstacles in the large distance and at the same time provide a high resolution.